Colour rendering reprographic apparatus, such as copiers and printers, apply image data with pixels that have a colour value. A colour value indicates the colour of a pixel and comprises several colour components or colour channels. A colour value is device dependent when its colour components induce different colours in different devices or in one device with different rendering conditions, such as in a printer with different types of paper, whereas a colour value is device independent if it specifies one and the same colour, independent of a device or its condition. Several device independent colour spaces, which are essentially coordinate systems for colour values, have been proposed for different purposes.
CIE (Commission International de l'Eclairage) colour spaces are device independent colour spaces using CIE Standard Observer functions that are based on colour matching functions and result in a unique set of tristimulus components XYZ for any colour measured under specified conditions. The tristimulus components XYZ are calculated from the spectral output of either an additive or subtractive colour system convoluted with a response function of either a 2 degree or 10 degree Standard Observer. In the case of a reflective hard copy, the spectral reflectance curve is typically convoluted with a standard illuminant, such as D50, to estimate the expected spectral output of the reflective colour. A very often used CIE colour space is the CIELAB colour space, wherein L* represents lightness, a* represents redness-greenness, and b* represents yellowness-blueness. CIELAB, which was established in 1976, employs a modified von Kries chromatic adaptation algorithm. Chromatic adaptation refers to perception of colour under different illuminants. According to the modified von Kries algorithm, chromatic adaptation involves dividing the tristimulus components XYZ by the white reference tristimulus components XwYwZw that are obtained for a perfectly diffuse white reflecting colour. In that way L*a*b* is (100,0,0) for the white reference colour under any illuminant. Therefore the colour with these coordinates is called neutral white. Similarly (0,0,0) is called neutral black and the axis in L*a*b* space between these points is called the neutral axis.
In order to render an image that is perceived similarly on various devices, each with its own colour space, the ICC (International Colour Consortium, www.icc.org) has created a standard environment for converting colour values. For this purpose so-called colour profiles are defined that comprise relations between colour values in a Profile Connection Space (PCS), which is a device independent colour space, and colour values in a device dependent colour space, such as RGB and CMYK values. An input device, such as a camera or a scanner, is characterized by an input colour profile, defining e.g. a relation between RGB and L*a*b*. An output device, such as a printer or a monitor, is characterized by an output colour profile, defining e.g. a relation between L*a*b* and CMYK or between L*a*b* and RGB. By combining the input and output profiles a conversion table for converting an image defined in the input colour space to an image defined in the output colour space is obtained.
The characterization, or profiling, of an output colour device occurs by measuring a number of predefined output colours under standard illuminating conditions. The production of these colours uses colour values in the device dependent colour space, whereas the measurement generates measurement values in a device independent colour space. Several commercial profiling software applications exist, such as Xrite/GretagMacbeth Profile Maker®, that derive a number of transforms, or rendering intents, that relate or associate these colour values in one colour space to colour values in the other colour space, thereby constituting an output profile. Each rendering intent, being one of four prescribed intents, has specific characteristics for transforming colours, particularly when colours are involved that are outside one of the colour gamuts.
For printers, a characteristic that is very important is the colour of the medium on which the image is reproduced. This is often a colour close to white, meaning that light of every wavelength in the visible range is to a substantial amount diffusively reflected. If, however, light with short wavelength is reflected somewhat less than other wavelengths, the medium may look yellowish, whereas if light with short wavelength is reflected somewhat more than other wavelengths, or if the medium comprises fluorescent material, the medium may look bluish. This is the shade of the medium.
By applying only black colorant on a medium having a shade, a monochrome hard copy is obtained. The medium without colorant is used to represent the pixels with the highest lightness in the image data. By the combination of pixels fully developed with black colorant and pixels with no colorant, gradations appear that are perceived as gray colours relative to the white medium, but colorimetrically these gray colours are not neutral. Therefore these gray colours are called medium gray colours to discern them from the neutral gray colours whose values are part of the neutral axis in the device independent colour space that was mentioned earlier. The darkest colour value occurs when all pixels in an area are developed with black colorant. In fact, as a function of the applied amount of pixels developed with black colorant, the lightness of gray areas varies from medium white to full black, defining a media gray axis in the colour space. The available lightness values in the image data may be scaled to the full range of gradations that the printer is able to reproduce. This gives the largest contrast in the image. Alternatively, the lightness is not scaled and a hard copy may result in which the most light image areas are reproduced with a small number of black pixels, which looks like background development. This is only used in special situations, e.g. for the reproduction of old photographs. The shade of the medium is not relevant for either monochrome hard copy.
When the same media is applied in colour printing, the shade of the medium affects the colours of areas on which a colorant is developed. Each rendering intent in an output colour profile accounts for the medium colour in its own specific way. An absolute rendering intent matches the input and output colours as accurately as possible. Therefore, when neutral white is the input colour, the absolute rendering intent compensates for the medium colour by developing some colorant of an opposite colour, e.g. a bluish medium takes a small amount of yellow colorant, in order that the output looks as much as possible like neutral white. The colorant that compensates the medium colour is similar to background colorant. A relative rendering intent however, uses the medium white as the white reference colour. Using this intent, neutral white is associated with no application of colorant. This may be viewed as a mapping of neutral white to the colour position of medium white. In the case of a bluish medium all colours are colorimetrically rendered more bluish and neutral white is rendered without background colorant, which is satisfying for many images. For other images the reproduction is inferior, because the colours do not look natural. No rendering intent is known that satisfies the different demands for good reproductions on different media with varying colour shades. Therefore in printing a hard copy there is a problem in combining the demands of having an output free of background colorant and natural colour rendering on arbitrary media.
An object of the present invention is to obviate the above problem.